The invention relates generally to a method of reducing exhaust emissions from two cycle gas engines and more specifically to a method of reducing nitric oxide and nitrogen dioxide of emissions from two cycle, turbocharged, natural gas, internal combustion engines.
Large bore, two cycle, internal combustion engines have proven their worth in many extended service applications requiring low r.p.m., high horsepower at stationary sites or in heavy equipment. Such engines are frequently used to power compressors on natural gas transmission lines, in diesel-electric locomotives or aboard ships. In such varied applications the engine is often adapted to utilize the most readily available or least expensive fuel. A pipeline compressor engine will operate on natural gas whereas a locomotive or ship engine will commonly operate on diesel fuel. The fuel adaptability of such engines is indicative of the sophistication of such engines in other areas.
Design and development of two cycle engines has been directed to many features and operational parameters of the engine. Understandably, much early research was devoted to improving the power output and efficiency of such engines. One approach attempted to ensure the complete evacuation of the products of combustion from the combustion chamber to encourage the ingestion of a maximum volume of fresh air and fuel. This evacuation or scavenging was found to be related to the location and size of the intake and exhaust ports as well as the configuration of the piston crown. U.S. Pat. No. 2,706,971 is directed to these considerations and to improvements therein.
Another area of development involved the combustion characteristics of an engine at low loads and low r.p.m. Generally speaking, the air/fuel ratio of an engine operating under low load conditions will be high, that is, the mixture will be lean. In most engines, this will create combustion difficulties since only exceedingly well mixed gas air volumes will ignite and sustain combustion properly. This problem becomes increasingly serious as the air/fuel ratio increases. In more conventional (i.e., small bore) engines, the relatively compact dimensions of the combustion chamber tend to minimize difficulties associated with complete mixing of lean combustion mixtures since the air and the gas are relatively easily uniformly distributed within the small volume of the combustion chamber.
In large bore internal combustion engines, which may typically have displacements of nearly four cubic feet, air/fuel mixing at high air/fuel ratios poses a difficult problem. Unchecked, the problem manifests itself as incomplete combustion, increased unburned hydrocarbon emissions and rough and irregular engine operation. The situation is further aggravated in gas pipeline compressor applications by the acknowledged necessity of such compressors to be driven at a constant speed. U.S. Pat. No. 2,723,653 is directed to the problem of low load operation of natural gas fueled engines.
U.S. Pat. No. 2,799,255 to Gehres discloses a two cycle engine incorporating a jet igniter cell. A small cell or secondary combustion chamber is in communication with the main combustion chamber and is independently supplied with a rich fuel mixture which is ignited by a spark plug controlled by the timing and ignition components of the engine. The hot gases from the jet cell in turn ignite the lean main combustion chamber mixture reliably and completely.
Other areas of development reflect contemporary economic conditions and trends. In 1965, when natural gas was abundant, price-controlled and therefore inexpensive, U.S. Pat. No. 3,187,728 disclosed a method and apparatus for converting a diesel engine to operate on natural gas. In 1978, the scarcity and presumed eventual de-regulation of the cost of natural gas has prompted research and development of a method and apparatus for converting a natural gas engine to operate on diesel fuel. U.S. Pat. No. 4,091,772 discloses such a configuration, now commonly owned application Ser. No. 686,279 filed May 14, 1976.
Until recently, one area of two cycle, large bore engine operation that had escaped scrutiny was exhaust emissions. Theoretical hydrocarbon chemistry explains that the combustion of natural gas which is constituted primarily of methane and ethane with oxygen produces carbon dioxide, carbon monoxide and water. However, since carbon, hydrogen and oxygen are not the only elements present during the combustion process, this theoretical explanation is accurate but not complete. Nitrogen, which constitutes approximately 80% of the atmosphere is also present and is affected by the combustion process. Nitrogen is present in the atmosphere in diatomic molecular form, designated N.sub.2. Since nitrogen is ingested into the combustion chamber with oxygen, it is present during the combustion process. The N.sub.2 molecules, heated by combustion, dissociate and combine with oxygen to form NO, nitric oxide, and NO.sub.2, nitrogen dioxide. (These two nitrogen-oxygen compounds are commonly lumped together and referred to as NO.sub.X.) These nitrogen bearing exhaust constituents have been identified as being highly deleterious to the atmosphere and living organisms and have thus become a target for minimization and elimination by internal combustion engine manufacturers.
A second group of exhaust constituents which have been targeted for reduction encompasses those products which devolve from incomplete combustion, such as unburned hydrocarbons and carbon monoxide. These exhaust constituents generally result from incomplete combustion due to incomplete air/fuel mixing and rich fuel/air mixtures.
Increasingly stringent emission standards promulgated by regulatory agencies of the federal and state governments have added immediacy to the search for engines and operating modes which lower total hydrocarbon and nitrogen related engine emissions.
One such search is described in Paper No. 71-WA/DGP-2 of the American Society of Mechanical Engineers. Published in 1971, the paper delineates research directed to the reduction of NO.sub.X emission in large bore diesel and natural gas engines. Operational parameters were individually varied to simulate a broad latitude of operating conditions. It was determined that NO.sub.X formation is sensitive to manifold temperature and air charging pressure. A reduction of NO.sub.X formation with increasing engine speed was also noted and attributed to the decreased residence time of the nitrogen gas within the combustion chamber at an elevated temperature during which the nitrogen might dissociate.
The indicated reduction of NO.sub.X and hydrocarbon emissions under high pressure, temperature and r.p.m. conditions did not, however, generate a pattern warranting further examination. In fact, certain operating data were recorded which ran precisely counter to the general trends described previously. The logical conclusion was that some unobserved parameter or interaction of parameters was affecting the emission performance of the engine in a fashion which was not then understood.